The proliferation of the Internet has created a need for large scale data centers that contain tens, if not hundreds, of racks of computing equipment, such as servers and routers. One of the major problems confronted by designers of these data centers is the requirement to route facility power to each of these racks of equipment. Typically, branch circuits from a primary or a secondary distribution panel are routed to groups of racks to provide power to the equipment in the rack. Each of the branch circuits is designed to provide a predetermined maximum power level or current, and the size of cabling used to route the power for a branch, and the size of circuit breakers used for the branch are selected based on this predetermined maximum power level or, more typically, predetermined maximum current.
Typically, it is desirable to design each of the branch circuits such that the total current drawn by the equipment coupled to any given one of the branch circuits is at some predetermined percentage (for example 50%) of the maximum current level for that branch circuit. This allows some flexibility to add additional equipment to racks and provides a safety margin below the maximum current level.
To properly design the routing of the branch circuits, it is desirable to know, with some accuracy, the current that is drawn by the equipment in the racks. Traditionally, the power or current drawn by computer equipment could be determined based on manufacturers' specifications and/or by making actual measurements of the current being drawn by the equipment.
These measurements and specifications are only useful for equipment for which the current draw is substantially static, which in the past was true for typical computing equipment. However, for newer computing equipment, the current draw is typically not static due to a number of factors including: 1) many computers utilize some form of power management strategy which minimizes the power (and current) consumption of the computer by turning off or slowing down subsystems within the computer when they are not in use; 2) cooling systems (i.e., fans) are often speed controlled based on air and component temperatures to reduce power consumption and acoustic noise generation; and 3) the amount of power drawn by the processors and memory systems in computers has increased steadily with the increase of speed of the processors, so that the power consumed by the processors and memory subsystems may exceed 50% of the total power draw of a computer. The power drawn by processors and memory systems is variable depending on the processing load, and since the total power of these systems may be a significant portion of the total power, the total power draw of a computer can vary significantly depending on the processing load on the computer.
The operating systems of most computers are capable of simultaneously performing multiple tasks by assigning segments of the CPU processing time to each of the tasks on a priority basis. Any remaining segments of the CPU processing time are occupied by an idle task in which the CPU can be halted and all associated clocks can be stopped to reduce the power draw of the computer. Further, some computers, for example, those that utilize the Windows® 98 or Windows® 2000 operating system, have an Advanced Control and Power Interface (ACPI) feature that allows the operating system to control power to fans and other devices in the computer to further reduce the power drawn by the computer. Because of the factors described above, it is not unusual for a more modern system to consume twice as much power when the processors are fully computationally loaded and operating in a warm environment, then when computationally idle and operating in a cool environment.
The variability of the power draw of computers complicates the electrical design of data centers. Computer manufacturers typically provide power ratings on nameplates. These nameplate values are typically maximum values that are determined based on the maximum power that a computer may draw when fully loaded with all options and with all subsystems operating at full load. Because of conservative approaches taken in determining nameplate values, they are often greater than even worst case values for a given computer, and accordingly are of little use to an electrical facility designer. While a designer may measure the current drawn by a computer or a set of computers to determine the power draw, it is typically not known at the measurement time, whether the computer is at full load or at what percentage of full load the computer is operating.
Several problems may occur when circuit branches are designed based on measured power draw values of computers. First, the wiring used in power routing circuits may be undersized for full load conditions, and second, when one or more of the computers powered from a branch are operated at full load, the current drawn may exceed the circuit breaker value for the branch, causing the circuit breaker to trip and disconnect power to the computers. For critical applications of computers, any such power interruption is often unacceptable. Further, to prevent power interruptions to critical computers, it is common to use uninterruptible power supplies (UPSs) for these computers. Often, one UPS is used to power multiple computers or racks of computers. To properly size the UPS, it is necessary to know the power draw of each of the computers and other equipment powered by the UPS. The variability of the power draw in newer computers makes it difficult to properly size a UPS for these applications.